1. Field of the Invention
The present invention relates to a process and an apparatus for producing entrapping immobilization pellets, entrapping immobilization pellets, and wastewater treatment equipment using the pellets. More particularly, the present invention relates to a process and an apparatus for producing entrapping immobilization pellets suitable for nitrification promoting advanced sewage treatment process in the field of wastewater treatment or the like.
2. Description of the Related Art
Nitrifying bacteria in activated sludge used for wastewater treatment grow slower than common bacteria. In particular, in winter during which water temperatures are low, such bacteria have only a small number of bacterial cells and thus exhibit significantly reduced nitrification activity. This tendency also applies to microorganisms having the same properties as in nitrifying bacteria. In this situation, there has been made an attempt to improve nitrification performance by attaching activated sludge containing nitrifying bacteria to the surface of an attachment material such as quartz sand, activated carbon, or plastic to increase the concentration of nitrifying bacteria (see “Water treatment by microorganism immobilization method: Pellet immobilization method, entrapping immobilization method, biologically activated carbon method,” published by NTS Inc. in 2000). However, attachment immobilization pellets in which microorganisms are attached to an attachment material have a drawback in that the concentration of nitrifying bacteria cannot be sufficiently increased because the attached microorganisms are released from the material, or microorganisms differing from nitrifying bacteria as target microorganisms are attached to the pellets. Accordingly, wastewater has been treated at high speed with increased nitrification activity by producing entrapping immobilization pellets in which useful microorganisms such as nitrifying bacteria are immobilized in an immobilizing agent, and packing a wastewater treatment tank with the entrapping immobilization pellets to increase the concentration of nitrifying bacteria.
Entrapping immobilization pellets are produced by tube forming, dropping granulation, sheet forming or the like. Tube forming is a method comprising injecting a mixture of microorganisms with a polymer material into a vinyl tube with a diameter of several millimeters to polymerize and extrude the mixture, and cutting the mixture into cylindrical pellets with a certain length. This method can provide pellets with high form accuracy, but is not suitable for mass production. Dropping granulation is a method comprising dropping a mixture of microorganisms with a polymer material into a separate liquid to produce spherical pellets. This method allows easy mass production, but has a drawback in that the produced pellets have various particle sizes.
As a device for solving these problems, Japanese Patent Application Laid-Open No. 2003-235553 discloses sheet forming comprising forming a mixture of microorganisms with a polymer material into a sheet, and finely cutting the sheet to produce rectangular pellets. In the sheet forming, as shown in FIG. 11, a raw material or chemical is cast into a space between belt conveyors 26, 28 by a stirring and extrusion unit 20, and is formed into a continuous sheet in the space between the belt conveyors. Then, the sheet is cut into lattices with a rotary blade 36 at certain intervals in the widthwise direction. Next, the lattice-shaped pellet block is cut into pellets 46 having a certain length using a guillotine cutting unit 38. This method can easily automate a granulation apparatus and provide a high form accuracy, and is thus assumed to be most suitable for mass granulation.